1. Field of the Invention
The present invention relates to a piezoelectric actuator.
2. Description of the Related Art
In a known piezoelectric actuator, a piston bore is formed in the actuator housing, a piston is slidably inserted into the piston bore, a variable volume chamber defined by the end face of the piston is formed on one side of the piston, a piezoelectric element is inserted between the actuator housing and the other side of the piston, and a belleville spring is mounted in the variable volume chamber to provide a compression load for the piezoelectric element via the piston (see Japanese Unexamined Patent Publication No. 59-206668). In this piezoelectric actuator, a stack type piezoelectric element made of a plurality of stacked piezoelectric element plates is used and when a charge is applied to the piezoelectric element, whereby the stack type piezoelectric element is expanded, the piston is moved to reduce the volume of the variable volume chamber, and accordingly, the pressure of fuel in the variable volume chamber is increased. When the charge in the stack type piezoelectric element is discharged, whereby the stack type piezoelectric element is contracted, the piston is moved to increase the volume of the variable volume chamber, and accordingly, the pressure of fuel in the variable volume chamber is reduced.
In such a stack type piezoelectric element , however, fine air gaps exist between the piezoelectric element plates, and therefore, when the piezoelectric element begins to expand and the pressure in the variable volume chamber is increased, the expansion of the piezoelectric element is stopped until the fine air gaps are eliminated. Namely, play exists in the piezoelectric element, and a sufficient expansion of the piezoelectric element cannot be obtained due to this play. In addition, to obtain a quick reduction of the fuel pressure in the variable volume chamber in response to the contraction of the piezoelectric element, the piezoelectric element must not move away from the piston. Therefore, in the above-mentioned piezoelectric actuator, a compression load is applied to the piezoelectric element by the belleville spring, via the piston, to eliminate the fine air gaps between the piezoelectric element plates, and as a result, when a charge is applied thereto, the required expansion of the piezoelectric element is obtained. Further, since the piezoelectric element does not move away from the piston when the piezoelectric element is contracted, the volume of the variable volume chamber can be instantaneously increased.
In another known piezoelectric actuator, a spring function is given to a hollow cylindrical member by forming, on the outer circumferential face thereof, a plurality of slots arranged around a point of symmetry in the transverse cross-section of the hollow cylindrical member, a stack type piezoelectric element having an outer diameter smaller than the inner diameter of the hollow cylindrical member is inserted into the interior of the hollow cylindrical member, and the stack type piezoelectric element is supported between plugs screwed into the opposed ends of the hollow cylindrical member (see Japanese Unexamined Patent Publication No. 58-106881). In this piezoelectric actuator, by applying a compression load to the stack type piezoelectric element by the spring force of the hollow cylindrical member, fine air gaps between the piezoelectric elements, and accordingly the play in the expansion of the piezoelectric element, are eliminated.
Where the belleville spring is used as shown in Japanese Unexamined patent publication No. 59-20668, however, since the belleville spring can not apply a uniformly distributed load to the piston, the piston is subjected to an eccentric load. Accordingly, due to this eccentric load imposed on the piston, the piston is inclined relative to the piston bore and a problem arises in that an eccentric wear of the piston will occur. In addition, if the piston is inclined relative to the piston bore, an eccentric load is imposed on the piezoelectric element, and thus another problem arises in that the piezoelectric element will be damaged. Furthermore, in the piezoelectric actuator shown in Japanese Unexamined Patent Publication No. 58-106881, since the hollow cylindrical member functioning as a spring is not supported in any way, the hollow cylindrical member is bent and offset from the center axis thereof, and as a result, an eccentric load is imposed on the piezoelectric element, and the problem of damage to the piezoelectric element arises.